140 GENERAL BIOLOGY 



Spore-formation. In many types of Protista 

 a modification of reproduction by fission is found, 

 which is especially advantageous in certain condi- 

 tions of existence. After repeated divisions of in- 

 dividuals in a free state, the organism may surround 

 itself with a thick wall, forming a cyst within which 

 the protoplasm fragments into a multitude of minute 

 particles called spores, each of which has the poten- 

 tiality of developing into an individual like that 

 which formed the cyst. In this way a species may 

 tide over a critical period, as of drouth, in such 

 an encysted condition. When favorable conditions 

 again intervene (perhaps after several years), the 

 multitude of emerging spores insure the immediate 

 existence of a large number of individuals, and thus 

 reduce the chances of extermination for the race. 

 In the bacteria, under certain conditions, the proto- 

 plasm of the tiny cell condenses into one or more 

 spores, which, on account of their minute size, may be 

 blown about in the dust, and thus afford the most 

 effective means for the dispersal of the species. 

 Propagation by spores is, indeed, a feature of devel- 

 opment throughout the plant kingdom. 



Plants are adapted in large measure to what we 

 call a " vegetative " existence ; that is, with the 

 exception of the Protophyta, most of which are 

 motile, they are fixed in the place where they sprout 

 and cannot seek food elsewhere if it fails, or avoid 

 extremes of climate by migrating. This disad- 

 vantage is compensated by the production of spores, 

 which are frequently developed in enormous numbers, 



